Ring size refers to the total number of signers in a ring signature. Before release 0.13.0 "Beryllium Bullet", it was possible to select an arbitrary number of signers. With release 0.13, it was decided to set this number to 11 to enforce transaction uniformity.
Ring size (11) = foreign outputs (10) + your output (1)
Foreign outputs are typically called "decoys," and the number of decoys was previously called the "mixin" size.